// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System;

namespace CommunityToolkit.Mvvm.ComponentModel;

/// <summary>
/// An attribute that indicates that a given type should have all the members from <see cref="ObservableRecipient"/>
/// generated into it. This can be useful when you want the same functionality from <see cref="ObservableRecipient"/> into
/// a class that already inherits from another one (since C# doesn't support multiple inheritance). This attribute will trigger
/// the source generator to just create the same APIs directly into the decorated class. For instance, this attribute can be
/// used to easily combine the functionality from both <see cref="ObservableValidator"/> and <see cref="ObservableRecipient"/>,
/// by using <see cref="ObservableValidator"/> as the base class and adding this attribute to the declared type.
/// <para>
/// This attribute can be used as follows:
/// <code>
/// [ObservableRecipient]
/// partial class MyViewModel : ObservableValidator
/// {
///     // Other members here...
/// }
/// </code>
/// </para>
/// And with this, the same APIs from <see cref="ObservableRecipient"/> will be available on this type as well.
/// <para>
/// To avoid conflicts with other APIs in types where the new members are being generated, constructors are only generated when the annotated
/// type doesn't have any explicit constructors being declared. If that is the case, the same constructors from <see cref="ObservableRecipient"/>
/// are emitted, with the accessibility adapted to that of the annotated type. Otherwise, they are skipped, so the type being annotated has the
/// responsibility of properly initializing the <see cref="ObservableRecipient.Messenger"/> property. Additionally, if the annotated type inherits
/// from <see cref="ObservableValidator"/>, the <see cref="ObservableRecipient.SetProperty{T}(ref T, T, bool, string?)"/> overloads will be skipped
/// as well, as they would conflict with the <see cref="ObservableValidator.SetProperty{T}(ref T, T, bool, string?)"/> methods.
/// </para>
/// </summary>
/// <remarks>
/// In order to work, <see cref="ObservableRecipientAttribute"/> needs to be applied to a type that inherits from
/// <see cref="ObservableObject"/> (either directly or indirectly), or to one decorated with <see cref="ObservableObjectAttribute"/>.
/// This is because the <see cref="ObservableRecipient"/> methods rely on some of the inherited members to work.
/// If this condition is not met, the code will fail to build.
/// </remarks>
[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
public sealed class ObservableRecipientAttribute : Attribute
{
}
